1. Field of the Invention
The present invention relates to conductive transparent thin films and in particular to a novel chemically stable co-doped zinc oxide material that has a low work function and exhibiting an increase of free electrons in the conduction band.
2. Description of Related Art
The present invention relates to conductive transparent thin films such as zinc oxide. It is well known that pure zinc oxide has low conductivity due to low electron density in the film and makes a poor conductor but, when doped, can exhibit improved electrical and optical properties. Zinc oxide is a common transparent n-type semiconductor material with a wide band gap of about 3.3 eV. Zinc oxide has been used as a thin film electrode in a variety of electrical applications such as solar cells or opto-electronics. It has a resistivity in the range of about 1xc3x9710xe2x88x924 to 1xc3x9710xe2x88x922 ohm -cm when doped, may be applied to a substrate using well-known semiconductor processing techniques and is relatively inexpensive when compared to other transparent oxide films such as indium tin oxide (ITO) or tin oxide.
Since zinc is a semiconductor material, researchers have investigated many methods for doping zinc oxide with a wide range of dopants. For example, non-stoichiometric zinc oxide compounds are known to have high conductivity and transmittance but, unfortunately, such compounds are electrically and optically unstable. It is also known that zinc oxide may be doped with Group VII elements such as fluorine or with Group III elements such as boron, aluminum, gallium or indium to obtain improved and stable electrical and optical properties. See for example, Jianhua Hu and Roy G. Gordon, Atmospheric pressure chemical vapor deposition of gallium doped zinc oxide thin film from diethyl zinc, water, and triethyl gallium, J. Appl. Phys., Vol. 72, No. 11, 1 December 1992.
Dopants are introduced to the zinc oxide using readily known semiconductor techniques. Such techniques typically require the high processing temperatures associated with semiconductor processing. This high temperature limits the selection of suitable substrates to substrates, such as glass, quartz or silicon, capable of withstanding the processing temperatures. Unfortunately, such substrates are expensive. Accordingly, it is desirable to develop a process that minimizes the processing temperature so that inexpensive plastic or other flexible substrates may be used.
One processing technique that has been used in the past is the ion implantation technique for implanting hydrogen, gallium, aluminum or boron ions at high energy into a zinc oxide film deposited on a glass substrate. It is well known, however, that ion implantation has a tendency to introduce damage to the lattice and create localized dislocations. Thus, an anneal step is also required after ion implantation to minimize the effect of the damage. Although the substrate temperature may be maintained at about 100xc2x0 C. during the implant process, the zinc oxide is changed from transparent clear to a yellowish-brown color even after annealing. See for example Shigemi Kohiki, Mikihiko Hishitani and Takahiro Wada, Enhanced electrical conductivity of zinc oxide thin films by. ion implantation of gallium, aluminum, and boron atoms, J. Appl. Phys., Vol. 75, No. 4, 15 February 1994.
Recent literature has also reported a number of attempts to impart electrical and optical properties to zinc oxide that would be comparable to ITO. By way of example, aluminum-doped zinc oxide thin film grown on sapphire, sputtering in argon (with hydrogen gas) added onto substrates of Corning 7059 glass, boron doped zinc oxide deposited on soda lime and quartz substrates, fluorine-doped zinc films deposited on soda lime glass substrates, argon ion beam sputtering in a hydrogen atmosphere, and ion implantation of gallium, aluminum and boron atoms in an attempt to improve conductivity of zinc oxide thin film. Other research has focused on undoped zinc oxide films having reduced resistivity brought about by treating the film surface with hydrogen in a mercury-sensitive photo-CVD process. Such zinc oxide thin films suffer from a variety of problems such as lacking chemical stability, having a hue other than transparent clear (i.e., yellowish), having poor transmissity and requiring high deposition or activation temperatures.
Growth, or deposition, of the thin film described in the prior art references is typically at a high temperature (between 100xc2x0 C. to 500xc2x0 C.) to activate the dopant and make the film conductive. Accordingly, the selection of an appropriate substrate is limited to a glass, quartz or silicon substrate able to withstand such temperatures. Further, subsequent processing may be required to achieve activation of the dopant by heating the film at high temperature after deposition. These high processing temperatures prohibit the use of many inexpensive flexible substrates such as plastic. Further still, while it is possible to obtain high conductivity with an appropriate dopant, the clarity of thin film is often not both clear and transparent. Alternatively, even if crystal clear and transparent, the zinc oxide film is not chemically stable.
Notwithstanding the improvements obtained by the above noted efforts, the processing techniques and the resulting zinc oxide thin film remain less than ideal for many electrical and optical applications. There exists great need for a zinc oxide material that is highly conductive, transparent, chemically stable and easily deposited on a variety of substrates, including flexible or plastic substrates.
As is well known to those skilled in the art, a common transparent conductive material used in many electrical and optical applications is indium tin oxide (ITO). ITO is considerably more expensive than zinc oxide but it has better electrical properties. Unfortunately, ITO also requires high processing temperatures to achieve the desired properties. What is needed is an improved, ITO material that may be activated at lower processing temperatures so that plastic or flexible substrates may be used instead of glass, quartz or silicon substrates.
The present invention relates to a novel zinc oxide material that is highly conductive, transparent, chemically stable and easily deposited on a variety of substrates, including flexible or plastic substrates, and is well suited for electrical or optical applications. The novel zinc oxide material is co-doped with two dopants, gallium (Ga) and hydrogen (H). A thin film is advantageously grown on a variety of substrates by ablation of a gallium-zinc oxide target in a gaseous hydrocarbon atmosphere. The co-doped zinc oxide thin film is transparent in the visible spectrum, conductive (0.9 to 3xc3x9710xe2x88x924 ohm-cm), has chemical and thermal stability and has a work function between 2.0 eV to 3.0 eV.
In one preferred embodiment, a sintered cylindrical-shaped target comprising a mixture of gallium and zinc oxide is positioned in a deposition chamber. The chamber atmosphere contains a mixture of oxygen-enriched atmosphere and a gaseous hydrocarbon such as methane, propane or ethane. The co-doped zinc oxide thin film is deposited using a pulsed laser deposition process although it is possible to use other deposition processes such as, by way of example, sputtering.
The co-doping of the zinc oxide with both gallium and hydrogen has significant advantages. Specifically, activation of the dopants at low growth temperature achieves a conductive and transparent zinc oxide film that can be deposited on a variety of substrates. The activation temperature is important since it is the minimum temperature to correctly position dopant atoms at the correct location in the lattice. As will be appreciated, a low deposition temperature typically leaves a very high percentage of the dopant atoms (i.e., gallium atoms) at interstitial locations of the lattice where such atoms do not contribute to the electron concentration or mobility. In such instances (e.g. prior art zinc oxide films), a high temperature anneal is required to replace the zinc atoms in the lattice with the gallium atoms. However, no such high temperature anneal is necessary to activate the dopant in the present invention since activation occurs even at low deposition temperatures. Further, the hydrogen atoms are incorporated into the zinc oxide most likely attached to the oxygen atoms. Every gallium atom in the lattice can stabilize at least one hydrogen atoms in the lattice. Advantageously, the high concentration of hydrogen does not disturb the lattice structure because of the small size of the hydrogen.
In the lattice of zinc oxide, a dopant can replace either a zinc atom or an oxygen atom. However, by co-doping with gallium and hydrogen, a gallium atom will replace a percentage of the zinc atoms in the lattice and the hydrogen appears to be absorbed on the oxygen side. Thus, by co-doping with two impurities, both sides of the lattice can contribute to the film conductivity. The result is a co-doped zinc oxide thin film with high electron concentration and high mobility compared to zinc oxide doped with a single dopant. Further, the high electron concentration of the co-doped zinc oxide thin film lifts the Fermi level to above the conduction band edge by an energy amount known as the Burnstein-Moss shift. Further still, the co-doped zinc oxide has a reduced work function that is less than 3 eV (more specifically, between 2 eV and 3 eV). Further still, the co-doped zinc oxide is crystal clear and transparent in the visible spectrum even when grown at relatively low processing temperatures (i.e., between about 30xc2x0 C. to 170xc2x0 C.).
In another preferred embodiment of the present invention, a novel indium tin oxide (ITO) thin film has the necessary electrical and optical properties that are obtained at low processing temperatures sufficient to permit the use of plastic or flexible substrates. The novel ITO comprises tin oxide co-doped with indium (In) and hydrogen.
Additional advantages to the present invention become apparent upon reading and understanding this specification.